Attila Nagy

The issue still occurs in 1.16.4, every quick flush destroys a grass block right at the source (it does not destroy the grass under flowing water). It should take a bit more time, it could have the same random mechanic with a short delay before it starts randomizing (like 3-4 seconds). That way the mechanic would stay, but we could use the flush based farms with full efficiency.

Same issue for me as well, however I haven't found (haven't searched for) the culprit block. I have a line on x coords which when I pass, it gives a huge lag spike.

1.6.4

edit: I've found the source for me, I hope it helps for some of you:
I have the "More Mob Heads" Vanilla Tweaks datapack added. Over the years I moved the map to a server, then to single player, then back to server, long story short, some mob heads are not displayed and I cannot fix it. I narrowed the lag source to one chunk, then I found that it was a tabby cat head, which wasn't displayed properly (Steve head instead)